Dont know if anyone posted these before:
Play MP3s on Wii: In the Picture Channel, start a slideshow with any number of pictures. Press 'A' for the options and change music. If you have any MP3s on your SD-card, they will show there. You can choose to play a lone MP3 or all. But exiting the slideshow will stop the music, sadly.
Wii-Sports, Tennis demo: When you setup the players in the game, make all the players CPU's (click on both of your Mii characters once and get a '?' instead of them). I have seen a 50-100 team against 1300-1500 pro team, but both teams appeared evenly matched against eachother. Some rallys between the teams will be 50 balls or more. At a certain number of balls played in the same rally/service, both sides will start to sweat a lot, as if they are tired because of all the running.